This invention relates to magnetic resonance imaging apparatus and more specifically to high current, gradient power supplies for use in such apparatus.
Magnetic resonance imaging ("MRI") has developed as an important tool in diagnostic medicine. In MRI, as is understood by those skilled in the art, a body being imaged is held within a uniform magnetic field oriented along a Z-axis of a Cartesian coordinate system.
The spins of the nuclei of the body are excited into precession about the z-axis by means of a radio frequency (RF) pulse. The decaying precession of these excited spins produces a nuclear magnetic resonance (NMR) signal whose amplitude is dependant, among other factors, on the number of precessing nuclei per volume within the imaged body. This number of spins is termed the "spin density".
An image of the spin density, or other characteristics revealed by the NMR signal, may be produced by impressing precisely controlled magnetic gradient fields G.sub.x, G.sub.y, and G.sub.z along the X, Y and Z axes. These gradient fields, created by gradient coils driven by a gradient amplifier system, encode position information into the NMR signals through phase and frequency shifting of the NMR signal for spins in different locations.
Referring to FIG. 1, a typical "spin echo" pulse sequence for acquiring data under the spin warp MRI technique includes: 1) a Z-axis gradient G.sub.z activated during a first 90.degree. RF pulse to select the image slice in the Z-axis, 2) a Y-axis gradient field G.sub.y to phase encode the precessing nuclear spins in the y direction, and 3) an X-axis gradient G.sub.x activated during the acquisition of the NMR signal to frequency encode the precessing nuclear spins in the x direction. Two such NMR acquisitions, S.sub.1 and S.sub.1 ', the latter inverted and summed with the first, comprise the NMR signal of a single view "A" under this sequence. Note that the y gradient field G.sub.y changes between view "A" and subsequent view "B". This pulse sequence is described in detail in U.S. Pat. No. 4,443,760, entitled: "Use of Phase Alternated RF Pulses to Eliminate Effects of Spurious Free Induction Decay Caused by Imperfect 180 Degree RF Pulses in NMR Imaging", and issued Apr. 17,1984, assigned to the same assignee as the present invention and incorporated by reference.
A set of NMR signals comprised of many views may be "reconstructed" to produce an image of a single slice of an imaged object according to well understood techniques. Multiple slices are needed to generate information over three dimensions of the imaged object.
The speed with which slice images may be obtained is limited, to a large extent, by the speed with which the gradient fields may be changed. The gradient coils are substantially inductive loads and hence obtaining higher speed switching of the gradient fields requires amplifiers capable of producing correspondingly higher voltages, often on the order of 2,000 volts. These higher voltages, together with the high currents required by the gradient coils (of 200 Amperes or more), demand amplifiers capable of extremely high power output.
The gradient amplifiers must also be capable of accurate control of the gradient current delivered to the gradient coils and should allow the maximum possible flexibility in the generation of gradient waveforms of arbitrary shape for present and future imaging techniques. For this reason, high powered linear amplifiers are most commonly used.
Previously, the power supply for a gradient coil utilized a single voltage inverter. Because of the relatively high voltages being switched, the single inverter had to use transistors capable of handling such voltages. It is desirable to be able to switch the high voltage with lower rated transistors.